In the area from the Culpeper basin on north, Early Jurassic ENA magmatism is well dated (~200 Ma), was of brief duration (<600 k.y.) and is manifested as synrift lava flows in all exposed basins and NE-striking diabase dikes. The minimum principal horizontal stress (SHmin) was oriented NW-SE, while SHmax was oriented NE-SW. Inversion structures post-date the ENA magmatism. For the southern basins, the age of the magmatism is more enigmatic; however, radiometric dating, paleomagnetism, geochemistry, and cross-cutting relationships indicate that the magmatism is likely Early Jurassic in age and was perhaps of somewhat longer duration than in the north. Synrift lava flows are absent, but postrift lavas have been identified in the subsurface. Diabase dikes strike N-S and NW-SE, and cut across the rift basins. SHmin was generally oriented NE-SW, and SHmax was generally oriented NW-SE. Inversion structures either pre-date or are broadly coeval with ENA magmatism.

Based on the above relations and assuming that the inversion structures were caused by ridge push and/or continental resistance to plate motion during incipient seafloor spreading, we propose the following tectonic history for eastern North America: (1) In the Late Triassic, the entire region experienced NW-SE extension. (2) In earliest Jurassic time (e.g., during ENA magmatism), rift basin subsidence had ceased in the south, and NW-SE extension was replaced by NW-SE shortening during the initial stages of interplate separation. Meanwhile, NW-SE intraplate extension, rifting, and basin subsidence continued in the north. (3) By the Middle Jurassic, seafloor spreading centers propagated northward, and the entire U.S. and SE Canadian Atlantic margin was undergoing postrift NW-SE shortening.

Go back to list of publications